1. Field of the Invention
This invention relates to remotely readable data storage devices and apparatus and particularly concerns such devices and apparatus which may be interrogated by an externally applied field, especially an alternating magnetic field.
2. Related Art
U.S. Pat. No. 4,510,490 (Anderson III et al) discloses a data storage device in the form of a tag for attachment to an article for identification purposes. The tag comprises a plurality of strips of magnetostrictive material which are of differing lengths to each other and accordingly have different natural i.e. fundamental frequencies. These strips are mounted in a casing sufficiently loosely that they are free to vibrate. The casing contains or includes a strip of hard magnetic material parallel to the magnetostrictive strips and magnetized so as to apply a biasing field thereto. The magnetostrictive strips are biased (stressed) by the field such that if an external alternating magnetic field of the natural frequency of any one of the strips is applied, that strip will resonate mechanically to produce a detectable alternating field at the natural frequency of the strip. Differing tags are distinguished from each other by different combinations of lengths of the magnetostrictive strips so that different tags will produce regenerated magnetic fields having different combinations of frequencies. Interrogation of a tag is performed by sweeping or stepping the interrogating magnetic field through the range of the natural frequencies of the strips or generating an alternating magnetic field as a burst containing all of those frequencies simultaneously, And detecting any regenerated fields. An application of this device to an article control system, such as for controlling luggage at airports, for example, is disclosed in U.S. Pat. No. 4,647,917 (Anderson III et al).
The above described proposals suffer from a number of disadvantages. Firstly, since each tag must contain a number of magnetostrictive strips equal to the number of "bits" of information to be stored, the tags are relatively expensive particularly if a large number of different tags are to be uniquely identifiable. Secondly, since the identity of a tag is defined by the lengths of the strips of magnetostrictive material incorporated in it, such identity has to be determined at the time of manufacture. This is extremely inconvenient and adds significantly to manufacturing cost.